It is the unit particle of pure substance. Hence, an elernent is defined as a chemically pure substance which can neither be decomposed into nor built from simpler substances by any kind of physical and chemical methods.
Hydrogen, oxygen, carbon, sodium, mercury, etc. are the e.rampies of elements. Altogether, there are 109 elements. Among them,92 elements are natually’obtained and remaining 17 elements have been prepared in laboratory by nuclear reactions.
Glassification of elements
Elements can be classitfied into three types
They are hearl solid substance and they possess the following character:
- They possess bright lustre (called metallic luster).
- They have high density melting and boiling point.
- They are very hard and have regular geometry
- They are malleable and ductile
- They are malleable and ductile
For example: Iron, Gold, Silver, Copper, Zinc, etc.
The chemically pure substances occurring in various states are non-metals. They possess the following characters
- They do not possess bright luster.
- They are poor conductors of heat and electricity.
- They generally possess low density, melting point and boiling point except diamond.
For example: Nitrogen, carbon, oxygen, sulphur e.t.c
They are the elements which have common characters of metals as well as non-metals. They are not as much as metals and non-metals in numbers.
For example, Arsenic, Antimony, Bismuth, etc.
Chemists worked to know the ultimate particle of an element and led to the discovery of atom. An atom may be defined as the smallest ultimate particle of an element which may or may not have independent existence but it always takes part in chemical combination. Atoms of some elements like hydrogen, oxygen and nitrogen do not have independent existence but atoms of Neon, Helium, Argon have independent existence.
Characteristics of Atom
- It can neither be created nor be destroyed.
- It is made up of electron, proton and neutron (except for hydrogen which is made up of electron and proton only).
- An atom is the unit of an element.
- It takes part in chemical reactions.
- Atoms generally do not exist in nature independently since they are reactive.
A compound is a pure substance which is obtained by the union of two or more elements in a definite proportion by weight which may be decomposed by suitable chemical methods. A compound has following identities:
- Compound is formed by the combination of two or more than two elements.
- Compounds can be chemically separated into their’ constituent elements.
- Compounds are formed by the chemical reaction.
- Properties of a compound is always different from its constituents.
- Compounds are chemically pure substances.
- Percentage of elements in a compound is always fixed.
Water – Compound of hydrogen and oxygen.
Ammonia – Compound of nitrogen and hydrogen.
Compounds are further divided into two types
The comporurds which are obtained from non-living sources such as rocks, minerals, etc.
Examples of the compounds are common salt, water, lime stone, etc.
They are the compounds rvhich are obtained from living sources, such as plants and animals. Carbon is an essential element in the organic compounds, such as carbohydrates, amino-acids, etc.
A molecule rs defined as the unit of a compor.rnd or group of sLmilar or dissrmilar atoms which has self-identity and erisls independently. A molecule should have folling characters.
- Ail molecuies of pure substances are similar in ali respect.
- They are chemically decomposable.
- Atoms are chemically bound with each other.
- Prope(ies of a molecule resembles the properties of the compound.
- They can exist in frec-state.